Density Altitude Explained: How Heat and Height Cut Performance

An aircraft performs as if it were at a different altitude when the air is hot, thin or both. That altitude is called density altitude. It is one of the most useful numbers to check before takeoff.

Three altitudes to know

Pressure altitude: the altitude shown when the altimeter is set to 1013.25 hPa (29.92 inHg).
ISA temperature: the standard temperature at that altitude. It starts at 15 °C at sea level and falls about 2 °C for every 1,000 ft.
Density altitude: pressure altitude corrected for the difference between the actual temperature and ISA.

The formulas

Pressure altitude ≈ field elevation + (1013.25 − QNH) × 27 ft
ISA temperature ≈ 15 − 1.98 × (pressure altitude ÷ 1,000)
Density altitude ≈ pressure altitude + 120 × (outside air temperature − ISA temperature)

Worked example

An airfield at 500 ft with QNH 1013 hPa and a temperature of 35 °C. Pressure altitude is about 507 ft. ISA temperature is about 14 °C, so the day is 21 °C hotter than standard. Density altitude is 507 + 120 × 21, about 3,030 ft. The aircraft behaves as if it were taking off from 3,000 ft.

What it does to the aircraft

Longer takeoff roll and landing distance.
Weaker rate and angle of climb, because engines make less power or thrust in thin air.
A higher true airspeed and ground speed for the same indicated airspeed.

Humid air is slightly less dense still. The risk is greatest on hot afternoons at high airfields, so check performance charts in your aircraft manual with the corrected value.

Open the density altitude calculator

FAQ

Does density altitude change during the day?

Yes. It usually peaks in the afternoon, when the temperature is highest, so morning departures often have better performance.

Is the density altitude formula exact?

No. This is the common rule-of-thumb approximation. Your aircraft manual's charts and tables are the reference.

More guides

For training and reference only. Check official sources before any real flight.